Lactic acid is widely applied even in an industrial use as a monomer raw material of biodegradable plastics other than a use for food, medicine and the like, and the demand is increasing. It is known that 2-hydroxypropionic acid, i.e., lactic acid is produced by a microbial fermentation, and the microorganism converts a substrate containing carbohydrate typified by glucose into lactic acid. Lactic acid is classified into optical isomers of (L)-isomer and (D)-isomer based on the configuration of a hydroxyl group bonded to a carbonyl carbon of α-position. Microbial fermentation can produce lactic acid of (L)-isomer or (D)-isomer selectively, or of a mixture (racemic body) of (L)-isomer and (D)-isomer by suitably choosing a microorganism.
The production of lactic acid by microbial fermentation is generally conducted while being maintained at an optimum pH for microbial fermentation by adding an alkaline substance to a culture solution. Lactic acid, an acidic substance produced by microbial fermentation, is mostly present as lactate in a culture solution by being added with an alkaline substance. In this case, free lactic acid is obtained by adding an acidic substance to a culture solution after completion of fermentation. Specifically, as one example of the alkaline substance to be added to a culture solution, calcium hydroxide is used, in this case, lactic acid produced by microbial fermentation is present as calcium lactate in a culture solution. Thereafter, by adding an acidic substance (for example, sulfuric acid) to a culture solution after completion of culture, a solution of free lactic acid can be obtained, but a calcium salt (for example, calcium sulfate) is produced as a by-product.
As a method for separating lactic acid by removing the calcium salt produced, in the case that a calcium salt is a poorly-soluble and precipitates like calcium sulfate, a method of filtering out through qualitative filter paper or the like is used. However, in the case of this method, a calcium salt that precipitates as a solid is removed, but a small amount of calcium salt dissolved in a solution is not removed, thus, it will remain in a lactic acid-containing solution. In the case that lactic acid is produced by microbial fermentation, various inorganic salts other than target lactic acid are produced as by-products, of which ones dissolved in a culture solution cannot be filtered out through qualitative filter paper or the like. Therefore, for example, when this filtrate containing lactic acid is subjected to a concentration procedure in the subsequent purification process, there arises a problem that calcium salt and other inorganic salts separate out (precipitation) again in a solution containing free lactic acid. When a lactic acid-containing solution is heated in operations such as distillation in a state that inorganic ions are not sufficiently removed, it is known that racemization and oligomerization of lactic acid proceed by the influence of inorganic ions. Therefore, a method for effectively removing a small amount of inorganic ion components remained in a lactic acid-containing solution is desirable.
As a method for removing a small amount of inorganic ion components from a lactic acid-containing solution, a method using an ion-exchange resin is disclosed (for example, see Japanese translation of PCT Publication No. 2001-506274). However, for maintaining ion-exchange performance of an ion-exchange resin, it is necessary to regenerate the ion-exchange resin periodically. In such regeneration of the ion-exchange resin, it is carried out using a large amount of sodium chloride aqueous solution, there is a problem that a lot of waste liquid is discharged being accompanied with regeneration, so that waste liquid treatment costs a lot. Further, there has been a problem that when an ion-exchange resin is regenerated repeatedly, the regeneration rate of an ion-exchange resin is lowered, in addition thereto, the ion-exchange performance is lowered, and the removal rate of inorganic salts is lowered.
There is also the known method that a small amount of inorganic ion components such as calcium component is removed from a lactic acid-containing solution by a bipolar membrane using an electrodialyzer (for example, see Japanese Unexamined Patent Publication No. 2005-270025). However, there has been a problem that the bipolar membrane used in this method is expensive, in addition thereto, the removal efficiency of inorganic salts such as calcium salt is by no means high.
In addition, a method of separation and recovery of organic acid using a separation membrane is known, and a method of separating para-pyruvic acid from a pyruvic acid solution through a reverse osmosis membrane is disclosed (see Japanese Unexamined Patent Publication No. 6-306011), but in that case, the removal rate of inorganic salts is not disclosed. Japanese Unexamined Patent Publication No. 6-306011 relates to a method of separation and recovery of pyruvic acid, but since pyruvic acid is not an optically-active material, problems in separation and recovery of lactic acid from a lactic acid-containing solution, namely, racemization and oligomerization have not been solved yet.
Further, a method of removing inorganic salts from a lactic acid-containing solution using a nano-filtration membrane is disclosed (for example, see U.S. Pat. No. 5,503,750, U.S. Pat. No. 5,681,728 and US 2004/0033573), but there is no disclosure on a process for recovery of lactic acid by distilling a lactic acid-containing solution obtained by filtrating a fermentation culture solution of a microorganism through a nano-filtration membrane, and on the influence that distillation affects the yield of lactic acid, so that the object of recovering lactic acid in a high yield by distillation has not been solved.
It could therefore be helpful to provide a method of effectively separating and recovering lactic acid in a culture solution while suppressing racemization and oligomerization.